JSF-JSP cancel button returns a number format exception

Question

I have a cancel button on a JSF-JSP page. It is handled by a method on the corresponding java class:

public String cancel_action() {
    return "CANCEL";

This is configured in faces-config.xml as following:

<navigation-rule>
    <from-view-id>/page2.jsp</from-view-id>
    <navigation-case>
        <from-outcome>CANCEL</from-outcome>
        <to-view-id>/page1.jsp</to-view-id>
    </navigation-case>
</navigation-rule>

Not sure why it is returning a number format exception.

Any help will be highly appreciated.